Surge protection device

ABSTRACT

A circuit protection device including a housing ( 15 ) defining a chamber ( 19 ) and a metal oxide varistor (MOV) stack ( 310 ) disposed within the chamber ( 19 ). A first spring ( 330   a ) is electrically attached at a first end to a first input terminal ( 311   a ) of the MOV stack ( 310 ) by a solder connection ( 30 ) and at a second end to a first input line ( 20   a ). The first spring ( 330   a ) is biased away from the first input terminal ( 311   a ). A second spring ( 330   b ) is electrically attached to a second input terminal ( 311   b ) of the MOV stack ( 310 ) by a solder connection ( 40 ) and at a second end to a second input line ( 20   b ). The second conductive spring ( 330   b ) is biased away from the second input terminal ( 311   b ). When an overvoltage condition occurs, heat generated by the MOV stack ( 310 ) melts at least one of the first or second solder connections ( 30, 40 ) to allow the corresponding springs to be displaced away from the respective MOV stack ( 310 ) input terminals ( 311   a,    311   b ), thereby creating an opening circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to the field of circuit protectiondevices. More particularly, the present invention relates to a surgeprotection device including a metal oxide varistor stack with anintegral thermal disconnect configured to provide an expedient thermalresponse in the event of overheating due to an abnormal overvoltagecondition.

2. Discussion of Related Art

Overvoltage protection devices are used to protect electronic circuitsand components from damage due to overvoltage fault conditions. Theseovervoltage protection devices may include metal oxide varistors (MOVs)that are connected between the circuits to be protected and a groundline. MOVs have a unique current-voltage characteristic that allows themto be used to protect such circuits against catastrophic voltage surges.These devices may utilize a thermal link which melts during anovervoltage condition to form an open circuit. In particular, when avoltage that is larger than the nominal or threshold voltage of an MOVis applied to the device, current flows through the MOV which generatesheat that causes the thermal link to melt. Once the link melts, an opencircuit is created which prevents the overvoltage condition fromdamaging the circuit to be protected. However, these existing circuitprotection devices do not provide an efficient heat transfer from theMOV to the thermal link, thereby delaying response times. In addition,MOV devices have relatively high inductance characteristics whichdegrade performance in the presence of fast overvoltage transients.Moreover, existing circuit protection devices are complicated toassemble and connect in certain applications such as, for example, inLED protection which increases manufacturing costs. Accordingly, it willbe appreciated that improvements are desirable in present day circuitprotection devices employing metal oxide varistors.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention are directed to a circuitprotection device. In an exemplary embodiment, the circuit protectiondevice includes a housing defining a chamber, a metal oxide varistorstack, and first and second conductive springs. The metal oxide varistorstack is disposed within the chamber of the housing. The firstconductive spring is electrically attached at a first end to a firstinput terminal of the metal oxide varistor stack by a first solderconnection and at a second end to a first input line. The firstconductive spring is biased away from the first input terminal of themetal oxide varistor stack. The second conductive spring is electricallyattached at a first end to a second input terminal of the metal oxidevaristor stack by a second solder connection and at a second end to asecond input line. The second conductive spring is biased away from thesecond input terminal of the metal oxide varistor stack, wherein when anovervoltage condition occurs, heat generated by the metal oxide varistorstack melts at least one of the first or second solder connections toallow the corresponding first or second conductive springs to bedisplaced away from the first or second input terminals of the metaloxide varistor circuit to define an open circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a perspective view of a circuit protection device inaccordance with an embodiment of the present disclosure.

FIG. 1 b is a schematic of a circuit protection device in accordancewith an embodiment of the present disclosure.

FIG. 2 a is a perspective view of a circuit protection device inaccordance with an embodiment of the present disclosure.

FIG. 2 b is a schematic of a circuit protection device in accordancewith an embodiment of the present disclosure.

FIG. 3 is a cut-away perspective view of a circuit protection deviceshown in an open condition in accordance with an embodiment of thepresent disclosure.

FIG. 4 is an exploded perspective view of a portion of the MOV stack andspring assembly shown in FIG. 3 in accordance with an embodiment of thepresent disclosure.

FIG. 5 a is a bottom perspective view of the circuit protection deviceshown in FIG. 1 a with the cavity of the bottom portion of the housingunfilled.

FIG. 5 b is a bottom perspective view of the circuit protection deviceshown in FIG. 1 a with the cavity of the bottom portion of the housingfilled with a potting material.

FIG. 6 a is a side perspective view of a first alternative embodiment ofthe circuit protection device of the present disclosure.

FIG. 6 b is a rear perspective view of the first alternative embodimentof the circuit protection device shown in FIG. 6 a.

FIG. 7 a is a side perspective view of a second alternative embodimentof the circuit protection device of the present disclosure.

FIG. 7 b is a rear perspective view of the second alternative embodimentof the circuit protection device shown in FIG. 7 a.

DESCRIPTION OF EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention, however, may be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, like numbers refer to like elements throughout.

In the following description and/or claims, the terms “on,” “overlying,”“disposed on” and “over” may be used in the following description andclaims. “On,” “overlying,” “disposed on” and “over” may be used toindicate that two or more elements are in direct physical contact witheach other. However, “on,”, “overlying,” “disposed on,” and over, mayalso mean that two or more elements are not in direct contact with eachother. For example, “over” may mean that one element is above anotherelement but not contact each other and may have another element orelements in between the two elements. Furthermore, the term “and/or” maymean “and”, it may mean “or”, it may mean “exclusive-or”, it may mean“one”, it may mean “some, but not all”, it may mean “neither”, and/or itmay mean “both”, although the scope of claimed subject matter is notlimited in this respect.

FIGS. 1A and 1B respectively illustrate a perspective view and aschematic diagram of a circuit protection device 10 in accordance withthe present disclosure. The protection device 10 may include a housing15, input lines 20 a, 20 b, and 20 c, and output lines 25 a and 25 b.The input line 20 a may be a line wire, the input line 20 b may beneutral, and the input line 20 c may be ground. Likewise, the outputline 25 b may be a corresponding line wire and the output line 25 b maybe a corresponding neutral. The input and output lines 20 a-c, 25 a, and25 b are used to connect the protection device 10 between a source ofpower (not shown) and a device or circuit to be protected (not shown) inaccordance with an embodiment of the present disclosure.

The housing 15 of the protection device 10 may be defined by an internalbottom portion 15 a (see FIG. 3) and a cover 15 b, where the bottomportion 15 a may include a plurality of flanges 17 extending therefromwhich may be used to fasten device 10 in an operative position (thebottom portion 15 a is referred to as “internal” because it is coveredby, and matingly fits within, the cover 15 b, such as by snap fit orfriction fit). The internal bottom portion 15 a and the cover 15 bdefine an enclosed chamber 19 within which a stack of metal oxidevaristors (MOV's) 35, 45, and 48 is disposed in electrical connectionwith input lines 20 a, 20 b, and 20 c and output lines 25 a and 25 b asfurther described below. Alternatively, it is contemplated that thebottom portion 15 a of the housing 15 can be an integral component orportion of a printed circuit board (PCB) of a device to be protected,and that the MOV's 35, 45, and 48, as well as the other internalcomponents of the protection device 10 (further described below) can bemounted directly to such component or portion of the PCB with the coverportion 15 b of the housing 15 fitting thereupon.

Referring briefly to FIG. 5 a, a bottom perspective view of the housing15 is illustrated where a plurality of apertures 21 may be formed in thesidewalls of the bottom portion 15 a of the housing 15 for allowing theinput lines 20 a-c and the output lines 25 a and 25 b to passtherethrough. The input and output lines 20 a-c, 25 a, and 25 b may thusextend into a cavity 27 that may be defined by the floor and sidewallsof the lower portion 15 a. The input and output lines 20 a-c, 25 a, and25 b may extend upwardly from the cavity 27, through the floor of thebottom portion 15 a, and into the chamber 19 (described above) forconnection with the internal components of the protection device 10 asdescribed below. Referring to FIG. 5 b, the cavity of the bottom portion15 a may be filled with a potting epoxy 29 or other solid or gelatinouscompound, such as a thermo-setting plastic or silicone rubber gel, usinga conventional potting process. The input and output lines 20 a-c, 25 a,and 25 b may thereby be encased in the potting material 29, which mayprovide protection against shock and vibration and prevent the ingressof moisture and corrosive agents which might otherwise damage ordeteriorate electrical connections between the lines and the protectiondevice 10. It will be appreciated by those of ordinary skill in the artthat the housing 15 may be embodied by a variety of alternativestructures and configurations that facilitate the electrical connectionsdescribed herein and that provide the MOV stack of the presentdisclosure (described below) with adequate protection from an externalenvironment.

Referring to FIG. 1B, a first thermal disconnect 30, such as may beformed of a low temperature solder fillet as further described below, isdisposed on the input line 20 a and is connected to one end of the firstMOV 35 via the output line 25 a. A second thermal disconnect 40, such asmay also be formed of a solder fillet, is disposed on the input line 20b and is connected to a second end of the MOV 35 and to a first end ofthe MOV 45 via the output line 25 b. A first end of the MOV 48 isconnected to the thermal disconnect 30 via the output line 25 a. Asecond end of the MOV 45 and a second end of the MOV 48 are connected tothe ground line 20 c. Thus, the MOV 48 is connected at a first end to afirst end of the MOV 35 and at a second end to ground and to the secondend of the MOV 45.

During normal operation of the circuit protection device 10 (i.e. wherean overvoltage condition does not exist), the stack of MOV's 35, 45, and48 does not produce a sufficient amount of heat to melt one or both ofthe thermal disconnects 30 and 40. However, since each of the MOV's 35,45, and 48 is a voltage sensitive device that heats-up when voltageapplied across the MOV exceeds the MOV's rated voltage, the occurrenceof an overvoltage condition causes the stack of MOV's 35, 45, and 48 toheat up. The heat radiated by the stack of MOV's 35, 45, and 48 upon theoccurrence of an overvoltage condition is sufficient to cause one orboth of the thermal disconnects 30 and 40 to melt, thereby creating anopen circuit which prevents the overvoltage condition from damaging adevice or circuit that is connected to the output lines 25 a and 25 b.

By the way of background, each of the MOVs 35, 45, and 48 may beprimarily comprised of zinc oxide granules that are sintered together toform a circular or square disc wherein the zinc oxide granule, as asolid, is a highly conductive material, while the intergranular boundaryformed of other oxides is highly resistive. Only at those points wherezinc oxide granules meet does sintering produce a ‘microvaristor’ whichis comparable to symmetrical zener diodes. The electrical behavior of ametal oxide varistor results from the number of microvaristors connectedin series or in parallel. The sintered body of an MOV also explains itshigh electrical load capacity which permits high absorption of energyand thus, exceptionally high surge current handling capability.

FIGS. 2A and 2B respectively illustrate a perspective view and aschematic diagram of an alternative embodiment of a circuit protectiondevice 200 in accordance with the present disclosure. The protectiondevice 200 may include a housing 215 and input connection lines 210 a,210 b, and 210 c. The input line 210 a may be a line wire, the inputline 210 b may be neutral, and the input line 210 c may be ground, forexample. The connection lines 210 a-c are used to connect the protectiondevice 200 between a source of power (not shown) and a device or circuitto be protected (not shown) in accordance with an embodiment of thepresent disclosure.

Similar to the housing 15 of the circuit protection device 10 describedabove and shown in FIGS. 1 a and 1 b, the housing 215 of the device 200may be defined by an internal bottom portion 215 a and a cover 215 b,where the bottom portion 215 a may include a plurality of flanges 217extending therefrom that may be used to fasten the device 200 in anoperative position (the bottom portion 215 a is referred to as“internal” because it is covered by, and matingly fits within, the cover215 b, such as by snap fit or friction fit). The bottom portion 215 aand the cover 215 b define an enclosed chamber 219 within which a stackof metal oxide varistors (MOV's) 235, 245, and 248 is disposed inelectrical connection with input lines 210 a-c as further describedbelow.

The bottom portion 215 a of the housing 215 may be provided withapertures 221 and a cavity (not within view) similar to those of thehousing 15 shown in FIG. 5 a for receiving the input lines 210 a-c. Thecavity of the bottom portion 215 a may be filled with a potting materialin a similar manner to the cavity 27 of the bottom portion 15 a of thehousing 15 described above and as shown in FIG. 5 b, wherein suchpotting material may include epoxy or other solid or gelatinouscompounds, such as a thermo-setting plastic or silicone rubber gel. Theinput lines 210 a-c may thereby be protected against shock, vibration,and moisture.

Referring to FIG. 2B, a first thermal disconnect 230, such as may beformed of a low temperature solder fillet as further described below,may be disposed on the connection line 210 a and may be connectedthereby to first ends of MOV's 235 and 248. A second thermal disconnect240, such as may also be formed of a low temperature solder fillet, maybe disposed on the connection line 210 b and may be connected thereby toa second end of the MOV 235 and to a first end of the MOV 245. A secondend of the MOV 245 and a second end of the MOV 248 may be connected tothe ground connection line 210 c. Thus, the MOV 248 is connected at afirst end to a first end of the MOV 235 and at a second end to ground.

During normal operation of the device 200 (i.e. where an overvoltagecondition does not exist), the stack of MOV's 235, 245, and 248 does notproduce a sufficient amount of heat to melt one or both of the thermaldisconnects 230 and 240. However, since each of the MOV's 235, 245, and248 is a voltage sensitive device that heats-up when voltage appliedacross the MOV exceeds the MOV's rated voltage, the occurrence of anovervoltage condition causes the stack of MOV's 235, 245, and 248 toheat up. The heat radiated by the stack of MOV's 235, 245, and 248 uponthe occurrence of an overvoltage condition causes one or both of thethermal disconnects 230 and 240 to melt, thereby creating an opencircuit which prevents the overvoltage condition from damaging a deviceor circuit that is protected by the device 200.

FIG. 3 is a cut-away perspective view of the exemplary circuitprotection device 10 shown in FIG. 1 with the cover 15 b removed fromthe housing 15. An MOV stack 310 is disposed within a portion of thechamber 19 defined by bottom portion 15 a. As described above, the MOVstack 310 may be comprised of a plurality of MOV's having associatedsurge capabilities and operating temperature ratings. For example, theMOV stack 310 may comprise three MOV's 35, 45, and 48 as described abovewith various pin configurations to accommodate connections to the inputlines 20 a-20 c and the output lines 25 a and 25 b. The MOV stack 310may be coated with epoxy and disposed within the chamber 19 of thebottom portion 15 a.

The input lines 20 a and 20 b may be connected to conductive springs 330a and 330 b that are mounted to the bottom portion 15 a of the housing15 in a cantilevered configuration. The non-cantilevered end of thefirst conductive spring 330 a may be connected to the input line 20 aand the non-cantilevered end of the second conductive spring 330 b maybe connected to the input line 20 b. The conductive springs 330 a and330 b may be connected to the input lines 20 a and 20 b via welding orother electrically-conductive connection means. The conductive springs330 a and 330 b may extend upwardly from their points of attachment tothe input lines 20 a and 20 b and may further extend at substantiallyright angles over respective protrusions 305 a and 305 b which extendupwardly from a top surface of the bottom portion 15 a. The protrusions305 a and 305 b serve to bias the cantilevered ends of each of therespective conductive springs 330 a and 330 b upwardly, away from afirst MOV connection terminal 311 a (not within view in FIG. 3 but shownin FIG. 4) and a second MOV connection terminal 311 b. Conductiveprotrusions 340 a and 340 b, which may be integral with the MOV stack310, may extend from the MOV stack 310 and may be connected to the inputwire 20 c to provide the MOV stack 310 with a connection to ground. Theoutput lines 25 a and 25 b may be connected to the first and secondoutput terminals 312 a and 312 b of the MOV stack 310.

As shown in FIG. 3, the conductive springs 330 a and 330 b of theprotective device 10 are in an open position, wherein the cantileveredends of the conductive springs 330 a and 330 b are not in contact withthe MOV connection terminals 311 a and 311 b, such as may be the caseafter the occurrence of an overvoltage condition. Conversely, in anormal operating condition, each of the respective conductive springs330 a and 330 b may be in a closed position, wherein the cantileveredends of the conductive springs 330 a and 330 b are electricallyconnected to their respective MOV connection terminals by lowtemperature solder fillets which define the thermal disconnects 30 and40 of the protection device 10 as described above.

FIG. 4 illustrates an exploded perspective view of a portion of the MOVstack 310 and the spring assembly shown in FIG. 3. As described abovewith reference to FIG. 3, the MOV stack 310 may include first and secondinput terminals 311 a and 311 b. As described above, the input terminals311 a and 311 b are electrically connected to the cantilevered ends ofthe first and second conductive springs 330 a and 330 b by lowtemperature solder fillets (i.e. thermal disconnects 30 and 40) when thecircuit protection device 10 is in a normal operating condition. Theinput line 20 a may be electrically connected to the non-cantileveredend of first spring 330 a. The input line 20 b may be electricallyconnected to the non-cantilevered end of second spring 330 b. The inputline 20 c may be electrically connected to the protrusions 340 a and 340b of the MOV stack 310. The output lines 25 a and 25 b may beelectrically connected to the output terminals 312 a and 312 b of theMOV stack 310, respectively.

Although both of the conductive springs 330 a and 330 b are illustratedas having a particular shape and configuration, many alternative shapesand configurations are contemplated and may be implemented in place ofthose shown and described above without departing from the presentdisclosure. For example, an alternative embodiment of the circuitprotection device 10 is contemplated in which the conductive springs 330a and 330 b are disposed within the chamber 19 and form connectionsbetween input lines 20 a-c and output lines 25 a and 25 b, respectively.

In another contemplated embodiment of the circuit protection device 10,shown in FIGS. 6 a and 6 b, the conductive springs 330 a and 330 b maybe connected to each other by a rigid, electrically insulating member600. The insulating member 600 substantially prevents relative movementof the conductive springs 330 a and 330 b. Thus, when the lowtemperature solder fillets (i.e. thermal disconnects 30 and 40 shown inFIG. 1 b) that connect the cantilevered ends of the conductive springs330 a and 330 b to the input terminals 311 a and 311 b melt, such asupon the occurrence of an overvoltage condition, the upwardly-biasedcantilevered ends simultaneously move out of contact with the inputterminals 311 a and 311 b, thereby simultaneously breaking theelectrical connections between the conductive springs 330 a and 330 band the input terminals 311 a and 311 b.

In yet another contemplated embodiment of the circuit protection device10, shown in FIGS. 7 a and 7 b, a high thermal conductivity,electrically insulating member 700 having electrically conductive,metalized end portions 700 a and 700 b may be interposed between thecantilevered ends of the conductive springs 330 a and 330 b and theinput terminals 311 a and 311 b. The insulating member 700 may be formedof any high thermal conductivity, electrically insulating material,including, but not limited to, aluminum nitride (AlN), beryllium oxide(BeO), silicon carbide (SiC), various metal-ceramic composites, or metalwith one or more insulative surface layers.

The metalized end portion 700 a of the insulating member 700 may beattached to the conductive spring 330 a by a high temperature bond, suchas a weld, and may be attached to the input terminal 311 a by a lowtemperature bond (i.e. thermal disconnect 30 shown in FIG. 1 b), such asa low temperature solder fillet. The term “high temperature bond” isdefined herein to mean a bond having a relatively higher meltingtemperature than that of the low temperature bond. Similarly, themetalized end portion 700 b of the insulating member 700 may be attachedto the conductive spring 330 b by a high temperature bond, such as aweld, and may be attached to the input terminal 311 b by a lowtemperature bond (i.e. thermal disconnect 40 shown in FIG. 1 b), such asa low temperature solder fillet. An electrical connection is therebyprovided between each of the conductive springs 330 a and 330 b and therespective input terminals 311 a and 311 b. Upon the occurrence of anovervoltage condition, the thermal conductivity of the electricallyinsulating member 700 allows heat to be transferred between the lowtemperature bonds that connect the conductive springs 330 a and 330 b tothe input terminals 311 a and 311 b, resulting in simultaneous meltingof the bonds and subsequent movement of the insulating member 700 awayfrom the input terminals 311 a and 311 b. The electrical connectionsbetween the conductive springs 330 a and 330 b and the respective inputterminals 311 a and 311 b are thereby simultaneously broken.

In view of the forgoing, it will be appreciated that a circuitprotection device in accordance with the present disclosure provides anexpedient thermal response in the event of overheating due to anabnormal overvoltage condition, and thereby effectively protects devicesor circuits that are connected to the circuit protection device fromdamage that could otherwise result from such overvoltage conditions. Inaddition, it will be appreciated that the circuit protection device inaccordance with the present disclosure may be implemented quickly,easily, and at relatively little cost relative to traditional circuitprotection devices that employ MOV's.

While the present invention has been disclosed with reference to certainembodiments, numerous modifications, alterations and changes to thedescribed embodiments are possible without departing from the sphere andscope of the present invention, as defined in the appended claims.Accordingly, it is intended that the present invention not be limited tothe described embodiments, but that it has the full scope defined by thelanguage of the following claims, and equivalents thereof.

What is claimed is:
 1. A circuit protection device comprising: a housingdefining a chamber; a metal oxide varistor stack disposed within thechamber; a first conductive spring electrically connected at a first endto a first input terminal of the metal oxide varistor stack by a firstthermal disconnect and at a second end to a first input line, whereinthe first conductive spring is biased away from the first input terminalof the metal oxide varistor stack; and a second conductive springelectrically connected at a first end to a second input terminal of themetal oxide varistor stack by a second thermal disconnect and at asecond end to a second input line, wherein the second conductive springis biased away from the second input terminal of the metal oxidevaristor stack, wherein when an overvoltage condition occurs, heatgenerated by the metal oxide varistor stack melts at least one of thefirst or second thermal disconnects to allow the corresponding first orsecond conductive springs to be displaced away from the first or secondinput terminals of the metal oxide varistor stack to define an opencircuit.
 2. The circuit protection device of claim 1, wherein the metaloxide varistor stack comprises a plurality of metal oxide varistors inan electrically parallel connection.
 3. The circuit protection device ofclaim 1, wherein the metal oxide varistor stack comprises a plurality ofmetal oxide varistors in an electrically serial connection.
 4. Thecircuit protection device of claim 1, wherein at least one of the firstand second thermal disconnects comprises a low temperature solderfillet.
 5. The circuit protection device of claim 1, wherein the housingincludes a first protrusion extending from a bottom portion of thehousing and configured to bias the first conductive spring away from thefirst input terminal of the metal oxide varistor stack.
 6. The circuitprotection device of claim 5, wherein the housing includes a secondprotrusion extending from a bottom portion of the housing and configuredto bias the second conductive spring away from the second input terminalof the metal oxide varistor stack.
 7. The circuit protection device ofclaim 1, wherein the housing comprises a bottom portion and a cover. 8.The circuit protection device of claim 7, wherein the bottom portion iscovered by, and matingly fits within, the cover.
 9. The circuitprotection device of claim 7, wherein the bottom portion of the housingdefines a cavity on a lower side thereof for receiving the input lines.10. The circuit protection device of claim 9, wherein the input linesextend from the cavity, through a floor of the bottom portion of thehousing, and into the chamber.
 11. The circuit protection device ofclaim 10, wherein the cavity in the bottom portion of the housing isfilled with potting material.
 12. The circuit protection device of claim11, wherein the potting material comprises an epoxy.
 13. The circuitprotection device of claim 11, wherein the potting material comprises athermo-setting plastic.
 14. The circuit protection device of claim 11,wherein the potting material comprises a silicone rubber gel.
 15. Thecircuit protection device of claim 1, further comprising a first outputline electrically connected to a first output terminal of the metaloxide varistor stack and a second output line electrically connected toa second output terminal of the metal oxide varistor stack.
 16. Thecircuit protection device of claim 1, further comprising at least oneconductive protrusion extending from the metal oxide varistor stack andelectrically connected to ground.
 17. The circuit protection device ofclaim 1, wherein the metal oxide varistor stack is coated with epoxy.18. The circuit protection device of claim 1, further comprising arigid, electrically insulating member connecting the first conductivespring to the second conductive spring and limiting relative movementtherebetween.
 19. The circuit protection device of claim 1, furthercomprising a thermally conductive, electrically insulating memberinterposed between the first and second conductive springs and the firstand second input terminals, the insulating member having a firstmetalized end electrically connected to the first conductive spring by ahigh temperature bond and electrically connected to the first inputterminal by a low temperature bond, and a second metalized endelectrically connected to the second conductive spring by a hightemperature bond and electrically connected to the second input terminalby a low temperature bond.